The thermal performance of vertical closed pulsating heat pipe (PHP) using pure water and hydroxylated MWNTs nanofluids as working fluids were experimentally investigated. The heat transfer characteristics of PHP are presented in terms of start-up procedure, temperature fluctuation and thermal resistance. Experimental results show that the MWNTs nanofluid PHP exhibits the better start-up characteristics than water as the concentration is below 0.3 wt%. The thermal characteristics are related to temperature fluctuation and a greater frequency of temperature fluctuation means more efficient heat transfer in the PHP. Furthermore, the results indicate that the heat transfer performance of PHP can be significantly improved by applying the hydroxylated MWNTs nanofluids at the lower concentration. When the input power increases up to 100 W, the thermal resistance of 0.1 wt% nanofluid PHP is decreased by 34% in comparison with pure water. In addition, a mathematical model based on the experimental data is developed to predict the velocity of fluids in the PHP. (C) 2017 Elsevier Ltd. All rights reserved.